1. Field of the Invention
The present invention relates to a motor control apparatus applied to the control of a stepping motor which is used as a driving source for a sheet conveying system of an image forming apparatus, an image forming apparatus provided with the motor control apparatus, a motor control method, and a program for implementing the method.
2. Description of the Related Art
Conventionally, in image forming apparatuses such as a copier, there has been widely used a sheet conveying mechanism in which DC motors or the like are used as driving sources for conveying sheets and driving forces of the DC motors are transmitted to a plurality of conveyor rollers arranged along a sheet conveying path via transmission mechanisms including gears and electromagnetic clutches to thereby convey sheets. In the sheet conveying mechanism, the driving of the conveyor rollers is controlled by opening and closing the electromagnetic clutches to thereby realize sheet conveyance control.
In recent years, with increasing demand for speedup of processing by image forming apparatuses, the speedup of the conveyance of sheets by the sheet conveying mechanism has been increasingly required. However, the conventional sheet conveyance control method of turning on/off the driving of the conveyor rollers by the use of the electromagnetic clutches or the like has a drawback that the response speed of the electromagnetic clutches is slow. This causes a bottleneck in realizing the speedup of the conveyance of sheets.
On the other hand, stepping motors have come to be widely employed as driving sources for a servo system that is small in size and can be controlled in open loop. The stepping motor is constructed such that exciting phase currents for exciting stator windings are sequentially switched to cause rotation of the magnetic field, which causes magnetic poles of a rotor to alternately attract and repel the stator windings to thereby generate torque, whereby the rotor is rotated. Therefore, if the switching of the exciting phases is carried out by inputting a pulse signal, the stepping motor is rotated through a basic angle whenever one pulse is input.
For this reason, open loop control is applicable to the stepping motor. When compared with other servo actuators requiring feedback control, a system including a stepping motor control mechanism can be significantly simplified, which is advantageous in cost.
Therefore, also in the field of image forming apparatuses such as a copier, there have appeared image forming apparatuses which incorporate a stepping motor control mechanism having stepping motors as driving sources, which are driven by a constant-current chopper control system. That is, in the image forming apparatuses of this type, stepping motors as many as the number of conveyor rollers are used as driving sources for the sheet conveying system, to drive the conveyor rollers without using electromagnetic clutches.
However, although the stepping motor can be designed compact in size and at low cost, a phenomenon occurs that the rotation of the rotor of the stepping motor cannot be synchronized with the input of a pulse signal, unlike conventional servo motors. This phenomenon is called “loss of synchronism”. In general, the loss of synchronism occurs when the stepping motor is in an overloaded state for the pulse rate of pulses outputted to the stepping motor from a driving circuit.
On the other hand, image forming apparatuses such as a copier are required to handle various types of sheets (plain sheet, thick sheet) and there is a case where torque required of the stepping motor largely varies depending on the type of sheet handled. Taking for example torque required when the sheet enters (a nip formed by) the conveyor rollers made of sponge and arranged along the sheet conveying path of the image forming apparatus, the torque required for the thick sheet (200 g/cm) can become 2 to 3 times as large as the torque required for the plain sheet (80 g/cm). Therefore, the selection of a stepping motor and the selection of the driving current for the stepping motor that determines output torque are carried out so as to cope with the thick sheet that usually requires such severe conditions.
Under the above described situations, to optimally control the stepping motor while avoiding loss of synchronism, for example, there has been proposed a technology in which, when thick sheets are conveyed, the distance between sheets is made larger than that in the case of conveying plain sheets to reduce torque applied to the stepping motor to thereby prevent loss of synchronism (for example, refer to Japanese Laid-Open Patent Publication (Kokai) No. 2001-310842).
Moreover, there has been proposed a technology in which, when thick sheets are conveyed, the driving current for the stepping motor is set to such a value as to output torque large enough to feed and convey the thick sheets to thereby prevent loss of synchronism (for example, refer to Japanese Laid-Open Patent Publication (Kokai) No. 2001-322734).
Furthermore, there has been proposed a technology in which thick sheets are conveyed at a rotational speed of the stepping motor slower than a rotational speed thereof for conveying plain sheets to reduce torque applied to the stepping motor and hence prevent loss of synchronism (for example, refer to Japanese Laid-Open Patent Publication (Kokai) No. 2002-211786).
On the other hand, in recent years, a feedback stepping motor has been developed as a new type stepping motor. The feedback stepping motor has provided therein a sensor for sensing a rotor position and monitors information on the rotational speed and the amount of rotation via the sensor during rotation as is the case with a servo motor, and when loss of synchronism is about to occur, immediately performs closed loop control, to thereby prevent occurrence of loss of synchronism even when the feedback stepping motor undergoes rapid load fluctuations or rapid acceleration.
However, the above conventional stepping motor control mechanism for image forming apparatuses has the following problems and hence there is demand for improvement to solve the problems.
As a typical method of using a stepping motor, it can be envisaged that when high torque is required only for a moment, the driving current setting of the stepping motor is variably controlled only for the moment. In general, the sheet conveying system of an image forming apparatus requires many (a dozen or so) stepping motors for driving many (a dozen or so) conveyor rollers and sheet feed rollers and hence it is necessary to control the set current value for each of the stepping motors in timing corresponding to each peak of the substantial torque on motor by motor basis and sheet by sheet basis, which results in complicated control of the stepping motors. Therefore, under the present circumstances, the driving current of each stepping motor has to be set in advance so as to cope with torque required under severe conditions.
As a result of such setting of the driving current, while the optimum torque can be outputted during the conveyance sequence of thick sheets, excessive torque that is larger than the required torque is outputted during the conveyance sequence of plain sheets. This causes a problem that the motors produce large vibration components, resulting adverse effects of noise. Moreover, currents larger than required amounts of current flow to the stepping motors during the conveyance sequence of plain sheets, resulting in the temperature rising high. Furthermore, the currents of the stepping motors are set in view of the case where the conveyance conditions are severe (stepping motors having a large current set range need to be used), which results in degraded efficiency and hence increased cost of the apparatus.
Moreover, the above-described technologies disclosed in Japanese Laid-Open Patent Publications (Kokai) Nos. 2001-310842 and 2002-211786 in which the distance between the sheets is made larger and the rotational speed is made slower, have a drawback that the number of sheets that can be subjected to image formation is reduced, thus leading to degraded productivity. Furthermore, the above-described technology disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2001-322734 has a drawback that when sheets of irregular types, that is, sheets other than the thick sheet and the plain sheet are conveyed, loss of synchronism cannot be prevented.
In addition, if the above-described feedback stepping motor can be used as a driving source for the sheet conveying system, no control is required for driving current setting, thus effectively preventing loss of synchronism. However, currently the feedback stepping motor is very expensive, so that it is not practical to employ such feedback stepping motors as many as the many (a dozen or so) conveyor rollers and sheet feed rollers constituting the sheet conveying system as the driving sources.